Radar system and method

ABSTRACT

A method for exciting an antenna with a waveform having a burst width and pulse width scaled proportionately with a selected range scale and a temporal filter to address any ambiguities in range resulting from the transmission of a signal in accordance with the novel waveform. The inventive filtering method includes the step of scanning a beam including a plurality of pulses of electromagnetic energy. The step of scanning the beam includes the step of outputting a beam excited by a waveform having a burst width and pulse width scaled proportionately with a selected range scale. Reflections of these pulses are received as return signals. The returns are processed to extract range in range rate measurements. The range and range rate measurements are compressed to form a plurality of range bins. The pulses are selectively weighted to reduce sidelobes resulting from a subsequent Fast Fourier transform (FFT) operation. The FFT operation is then performed for a predetermined number of pulses in at least one of the range bins at at least one frequency. A second FFT operation is then performed for pixels of azimuth data across the range bins. Finally, ambiguity nulling weights are provided and applied to each pixel of data in each range bin.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to imaging systems. Morespecifically, the present invention relates to radar imaging systems.

[0003] 2. Description of the Related Art

[0004] Imaging techniques are well known and widely used in the art.Certain imaging technologies are better suited for particularapplications. For example, radar imagery is widely used for surveillanceand reconnaissance as well as target tracking and identification. Forradar and other imaging technologies, the ability to clearly resolve anddiscriminate targets may be essential in meeting objectives specifiedfor a particular application.

[0005] One such application involves ‘real beam ground mapping’. Realbeam ground mapping involves scanning an area, e.g., the earth'ssurface, using a scanning antenna or an electronically scanned antenna.Returns from an illumination of the surface are then examined for‘back-scatter’ or reflections therefrom. As the beam is scanned inazimuth, information is collected with respect to the range direction.At each beam position, the distance of various scatterers may beascertained for each range cell. This information may then be displayedin a real beam ground mapped image.

[0006] Unfortunately, at large side-looking (azimuthal) scan anglesrelative to the velocity vector, the Doppler spectrum of the clutterspreads out significantly over many Doppler filters relative to theDoppler spectrum of signals received closer to the direction of thevelocity vector of the platform over the entire pulse repetitionfrequency (PRF). In a Real Beam Ground Mapping application, this maylead to the creation of maps with poor image quality.

[0007] While range data may be resolved with adequate resolution,currently, resolution of azimuth data with comparable resolution hasproved to be problematic. This is due to the fact that azimuthresolution is limited to the width of the beam and degrades as afunction of range. Accordingly, the poor resolution of conventional realbeam mapping systems limits the ability of the system to discriminatescatterers.

[0008] SAR (synthetic aperture radar) has been used for ground mapping.However, currently, SARs require several seconds at each beam positionand are therefore too slow for many more demanding (e.g., military)applications.

[0009] “Super resolution” techniques are widely used to sharpen theradar imagery. However, the quality achieved is scene dependent and isnot robust. The current techniques do not effectively account for theimpact of the radar system on the true scene.

[0010] Hence, a need remained in the art for an improved system ormethod for providing ground mapped images. Specifically, a need remainedin the art for a system or method for providing enhanced cross-range(azimuthal) resolution for a real beam ground mapping radar system. Thisneed was met by copending application entitled RADAR IMAGING SYSTEM ANDMETHOD, filed ______ by Kapriel V. Krikofian and Robert. A. Rosen, Ser.No. ______, (hereinafter the “Krikorian et al system”) the teachings ofwhich are incorporated herein by reference.

[0011] Notwithstanding the fact that Krikorian et al substantiallyaddressed the need for a system for providing enhanced cross-rangeresolution, an additional problem remains with respect to range andDoppler ambiguity. That is, when a conventional radar system searches inthe cross-scan direction for unambiguous returns within the range of,say, 60 miles, the pulse repetition frequency (PRF) must be low. Thatis, the pulse repetition interval (PRI), which is equal to the pulsewidth plus the desired range, must be greater than 60 miles to cover theentire distance without ambiguity. Thus, inasmuch as the PRF is theinverse of the PRI, with a long PRI, the PRF must be low. If not, theDoppler spectrum becomes ambiguous and scatter returns from spurioussources of reflection begin to fall in the same range bins on top ofeach other. This requires processing of the returns from each pulseseparately and precludes a desirable coherent integration of same. Theonly current option then is to integrate the pulses non-coherently bysimply summing the magnitudes thereof. Unfortunately, the resulting realbeam ground maps reflect the loss of sensitivity associated with thisapproach.

[0012] Hence, a need remains in the art for an improved system or methodfor providing real beam ground mapped images. Specifically, a needremains in the art for a system or method for long range real beamground mapping with improved sensitivity at high azimuth look angles.

SUMMARY OF THE INVENTION

[0013] The need in the art is addressed by the system and dataprocessing methods of the present invention. There are at least twosignificant aspects of the invention. One is the provision of a methodfor exciting an antenna with a waveform having a burst width and pulsewidth scaled proportionately with a selected range scale. The second isthe provision of a temporal filter to address any ambiguities in rangeresulting from the transmission of a signal in accordance with the novelwaveform.

[0014] The inventive filtering method includes the step of scanning abeam including a plurality of pulses of electromagnetic energy.Reflections of these pulses are received as return signals. The returnsare processed to extract range in range rate measurements. The range andrange rate measurements are compressed to form a plurality of rangebins. The pulses are selectively weighted to reduce sidelobes resultingfrom a subsequent Fast Fourier transform (FFT) operation. The FFToperation is then performed for a predetermined number of pulses in atleast one of the range bins at least one frequency. A second FFToperation is then performed for pixels of azimuth data across the rangebins. Finally, ambiguity nulling weights are provided and applied toeach pixel of data in each range bin.

[0015] In illustrative embodiments, the step of scanning the beamincludes the step of outputting a beam excited by a waveform having aburst width and pulse width scaled proportionately with a selected rangescale. The step of selecting and weighting return pulses in the rangebins includes the step of selecting and weighting the return pulses toreduce sidelobes resulting from the Fast Fourier transform of thepredetermined number of pulses. The step of selecting and weightingreturn pulses in the range bins includes the step of selecting thepulses based on antenna scan weight and the pulse repetition frequencyof the pulses.

[0016] Further, in the illustrative embodiment, the step of performing aFast Fourier Transform for pixels of azimuth data across the range binsincludes the steps of selecting Fast Fourier Transform weighting windowsand performing a fast Fourier transform for pixels of azimuth dataacross the range bins based on scan geometry and history of the scanbeam.

[0017] In the illustrative embodiment, the step of applying nullingweights to each pixel of data in each range bin includes the step ofapplying nulling weights to each pixel of data in each range bin basedon beam scan geometry, scan history, range, range ambiguity, and PRFDoppler ambiguity.

[0018] Further, in the illustrative embodiment, the step of applyingnulling weights to each pixel of data in each range bin further includesthe step of applying nulling weights to each pixel of data in each rangebin at each of a plurality of predetermined frequencies. Theillustrative embodiment of the inventive method further includes thestep of performing pulse detection integration across each of thefrequencies of the beam.

[0019] The inventive system may be implemented in software running on aprocessor. In a specific implementation, the system generates a novelradar waveform which effects a higher duty factor and provides bettersensitivity. An additional novel aspect of the invention is theprovision of a temporal filter in the azimuth direction to significantlyreduce Doppler ambiguity.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020]FIG. 1 is a generalized block diagram of a radar systemimplemented in accordance with the teachings of the present invention.

[0021]FIG. 2 is a diagram of a waveform generated in accordance with anillustrative embodiment of the method of the present invention.

[0022]FIG. 3 is a flow diagram showing an illustrative embodiment ofsoftware used to implement a temporal filter within the teachings of thepresent invention.

[0023]FIG. 4 is a diagram showing a return from a clutter patch ofinterest and the nearest ambiguities with respect to the illustrativetemporal filtering process implemented in accordance with the teachingsof the present invention.

[0024]FIG. 5 shows overlapping FFT amplitude weighting windows coveringthe region between the nearest Doppler ambiguities with respect to theillustrative temporal filtering process implemented in accordance withthe teachings of the present invention.

[0025]FIG. 6 shows the amplitude response of the main clutter patch andthe ambiguous patches in the Doppler filter containing the clutter patchof interest with respect to the illustrative temporal filtering processimplemented in accordance with the teachings of the present invention.

[0026]FIG. 7 is a diagram showing ambiguity cancellation weights acrossFFT arrays with respect to the illustrative temporal filtering processimplemented in accordance with the teachings of the present invention.

[0027]FIG. 8 is a diagram which shows a time domain representation ofillustrative weights for a Doppler FFT operation in accordance with thepresent teachings.

[0028]FIG. 9 shows two graphs as a function of time: one being anillustrative response of a DC Doppler filter to a particular scattererand the other being a response of the same Doppler filter to anambiguous scatterer.

[0029]FIG. 10 is a diagram showing illustrative weights for the temporalfilter as a function of time in accordance with the teachings of thepresent invention.

DESCRIPTION OF THE INVENTION

[0030] Illustrative embodiments and exemplary applications will now bedescribed with reference to the accompanying drawings to disclose theadvantageous teachings of the present invention.

[0031] While the present invention is described herein with reference toillustrative embodiments for particular applications, it should beunderstood that the invention is not limited thereto. Those havingordinary skill in the art and access to the teachings provided hereinwill recognize additional modifications, applications, and embodimentswithin the scope thereof and additional fields in which the presentinvention would be of significant utility.

[0032]FIG. 1 is a generalized block diagram of a radar systemimplemented in accordance with the teachings of the present invention.Those skilled in the art will appreciate that although the presentteachings are disclosed with reference to an illustrative radar systemimplementation, the invention is not limited thereto. The presentteachings may be applied to a variety of image processing applicationswithout departing from the scope thereof. The system 100 includes areceiver/exciter 101 of conventional design and construction. As isknown in the art, the receiver/exciter 101 includes an exciter/waveformgenerator 102 that generates a novel and advantageous waveform asdiscussed more fully below. The radar signal is upconverted by anupconvert stage 104 and filtered, amplified and transmitted by atransmitter stage 108 in response to a reference signal from a modulator106. The transmit signal is radiated by a radar antenna 110 as a beam ofelectromagnetic energy.

[0033] In an illustrative real beam ground mapping application, scatterreturns of the transmit beam as it is reflected from the ground or othersurface are received by the antenna 110 and applied to a radar receiverstage 112. The receiver 112 amplifies, filters and downconverts thescatter return in a conventional manner. The convolved, amplified,filtered and downconverted scatter returns are digitized by ananalog-to-digital converter stage 114 and fed to a processor 116 in aprocessor stage 115.

[0034] In accordance with the present teachings, in the illustrativeembodiment, processor 116 feeds a signal to the exciter/waveformgenerator 102 effective to cause the generator 102 to output a waveformsuch as that shown in FIG. 2. In accordance with the present teachings,the intra-burst PRF and pulse width are selected as a function of theburst width (i.e., the minimum mapping range) to maximize the dutyfactor within the transmitter or array driver performance limits.

[0035]FIG. 2 is a diagram of a waveform generated in accordance with anillustrative embodiment of the method of the present invention. Thewaveform features a low of pulse repetition frequency (PRF) with a burstwidth and pulse width scaled proportionately with the selected rangescale. In accordance with the present teachings, the intra-burst PRF andpulse width are selected as a function of the burst width (i.e. theminimum mapping range) to maximize the duty factor within thetransmitter or array driver performance limits. The duty factor shouldbe high, e.g.:

d _(T)=(1/3)d _(B)  [1]

[0036] where d_(B)=burst duty factor=0.5 (or as high as possible) e.g.,d_(B)=16.7%.

[0037] In accordance with the present teachings, the chirp bandwidth isinversely proportional to range scale (i.e., maximum range R_(MAX)):

S=c/(2·R_(MAX) ·q)  [2]

[0038] where ‘c’ is the speed of light and ‘q’ is the fractional rangeresolution.

[0039] Those skilled in the art will appreciate that this waveformprovides a high duty factor with frequency agility within each burst. Atlong range, the average duty factor, and thus the sensitivity, is higherfor a burst of pulses than for a conventional waveform with a singlepulse per PRI. This is because a long PRI is required to avoideclipsing. Thus, a high duty factor with a conventional waveform wouldrequire a long pulse, which would exceed the capacity of the arraydriver.

[0040] Intra-burst modulated pulses for high range resolution atdifferent radio frequencies (frequency agility) should be used to reducescintillation. In accordance with the present teachings, low PRFburst-to-burst coherency is maintained for Doppler processing on eachradio frequency to achieve fine cross-range resolution. At long rangeoperation, the low burst-to-burst PRF results in Doppler ambiguities athigh scan angles from the velocity vector. In accordance with thepresent teachings, the Doppler ambiguities may be reduced by a noveltemporal filtering technique that combines radar returns from thedifferent beam positions of a mechanically or electronically scannedantenna. The temporal filtering technique is implemented in software bythe processor 116. An illustrative embodiment of the software isillustrated in FIG. 3.

[0041]FIG. 3 is a flow diagram showing an illustrative embodiment ofsoftware used to implement a temporal filter within the teachings of thepresent invention. As shown in FIG. 3, the inventive method 200 includesthe step 202 of exciting the antenna 110 of FIG. 1 with the waveformshown in FIG. 2. A beam output by the antenna 110 is reflected by one ormore targets and the resulting returns are received at step 204. Thereturns are processed by the system 100 of FIG. 1 and output as aplurality of range and range rate measurements. At step 206, anintra-pulse measurement (range compression) is performed to form aplurality of range bins as is common in the art. Next, at step 208, thesoftware sets up and executes a control loop over a plurality of FFTweighting windows. The first step in the control loop involves the step210 of selecting and weighting pulses to reduce sidelobes resulting fromFFT filtering as is common in the art. At step 212, the software 200performs several FFTs within one range bin and one radio frequency basedon the PRF and scan rate of the beam. This process is repeated for eachrange bin and each radio frequency.

[0042] Next, the software 200 sets up and enters a control loop overazimuth pixels. The first step in this loop 218 involves a selection ofFFT weighting windows and FFT filtering based on scan geometry and scanhistory. Next, at step 220, ambiguity nulling weights are selected basedon scan geometry and scan history. In accordance with the presentteachings, the weights are chosen to compensate for range ambiguity andPRF Doppler ambiguity. At step 222, for each range bin the nullingweights are applied to the FFT outputs and the weighted outputs areadded across each of the FFT weighting windows. At step 224, a pulsedetection integration is performed across each radio frequency and theentire process is repeated until all of the azimuth pixels have beenprocessed.

[0043] This process is best illustrated with respect the followingexample: Consider a radar with a nose mounted electronically scannedantenna of frequency f_(o)=GH_(z), 3 dB beamwidth of θ_(o), =3 deg, anownship velocity v=200 m/s and a pulse repetition frequency PRF=1 KHz.The clutter Doppler frequency is given by:

f _(c)=2f _(o)ν cos θ/c  [3]

[0044] where θ is the azimuth angle relative to the ownship velocityvector.

[0045] The azimuth angles of the Doppler ambiguity are given by:$\begin{matrix}{\theta_{amb} = {\cos^{- 1} - \frac{\left( {f_{c} + {kPRF}} \right)c}{2f_{o}v}}} & \lbrack 4\rbrack\end{matrix}$

[0046] where k=±1, ±2, . . . .

[0047] At a small azimuth angle of 0=10°, the nearest ambiguity is at24.5° which falls in the far sidelobe region of the antenna and thus hasan insignificant effect. However, at a larger azimuth angle of θ=60°,the nearest ambiguities are at 64.9 and 54.8° which are within theboundaries of the broadened antenna mainlobe. The outer ambiguities havean insignificant effect because of the rejection imposed by the lowantenna sidelobes.

[0048]FIG. 4 is a diagram showing a return from a clutter patch ofinterest and the nearest ambiguities with respect to the illustrativetemporal filtering process implemented in accordance with the teachingsof the present invention. As shown in FIG. 4, as the beam is scannedpast the clutter patch of interest, the return from the patch and thetwo ambiguities is as shown. Overlapping weighted pulse to pulse FFTsover a scanning beam are performed.

[0049]FIG. 5 shows 5 overlapping FFT amplitude weighting windowscovering the region between the nearest Doppler ambiguities.

[0050]FIG. 6 shows the amplitude response of the main clutter patch andthe ambiguous patches in the Doppler filter containing the clutter patchof interest. As shown, the response varies differently for the ambiguouspatches across the overlapping FFTs.

[0051]FIG. 7 is a diagram showing ambiguity cancellation weights acrossFFT arrays with respect to the illustrative temporal filtering processimplemented in accordance with the teachings of the present invention.The corresponding set of weights that cancel the ambiguity of theclutter patches at 64.9 and 54.8 degrees is shown in FIG. 7.

[0052] FIGS. 8-10 are diagrams which illustrate the temporal filteringtechnique for a scanning beam of the present invention.

[0053]FIG. 8 is a diagram which shows a time domain representation ofillustrative weights for a Doppler FFT operation in accordance with thepresent teachings.

[0054]FIG. 9 shows two graphs as a function of time: one being anillustrative response of a DC Doppler filter to a particular scatterer300 and the other being a response of the same Doppler filter to anambiguous scatterer 310.

[0055]FIG. 10 of the diagram showing illustrative weights for thetemporal filter as a function of time in accordance with the teachingsof the present invention. The technique illustrated in FIG. 3 nulls outDoppler ambiguities and applies to the regions of the real beam radarimage that has a high azimuth angle relative to the velocity vector. Theextent of this region depends on the scan width, aircraft speed, andrange. In accordance with the present teachings, the temporal filteringis performed at high scan angles from the velocity vector (e.g.,typically >40 deg) where the clutter spreading is larger and higherburst to burst PRFs would otherwise be required to reject Dopplerambiguities. At each beam position, a weighted burst-to-burst FFT isperformed. As the beam scans the mainlobe of the same Doppler filterresponds more strongly to ambiguous scatterers which corrupted theoriginal beam position. By applying temporal weights and combining thesame Doppler filter at each beam position of a scanning beam, theDoppler ambiguities are reduced. This technique allows utilization oflow burst PRFs to achieve true cross range resolution (e.g. 20:1 beamsharpening) with a fast scanning beam at long ranges and high scanangles. This technique does not compromise the fast scan rate requiredfor the radar real beam ground map (RBGM) mode.

[0056] Thus, the present invention has been described herein withreference to a particular embodiment for a particular application. Thosehaving ordinary skill in the art and access to the present teachingswill recognize additional modifications applications and embodimentswithin the scope thereof.

[0057] It is therefore intended by the appended claims to cover any andall such applications, modifications and embodiments within the scope ofthe present invention.

[0058] Accordingly,

What is claimed is:
 1. A radar system comprising: an antenna and meansfor exciting said antenna with a waveform having a burst width and pulsewidth scaled proportionately with a selected range scale.
 2. Theinvention of claim 1 wherein an intra-burst PRF is selected as afunction of a burst width to maximize a duty factor.
 3. The invention ofclaim 2 wherein the duty factor is d _(T)=(1/3)d _(B)  [1]whered_(B)=burst duty factor.
 4. The invention of claim 1 wherein anintra-burst pulse width is selected as a function of a burst width tomaximize a duty factor.
 5. The invention of claim 4 wherein the dutyfactor is d _(T)=(1/3)d _(B)  [1]where d_(B)=burst duty factor.
 6. Theinvention of claim 1 wherein a chirp band width is inverselyproportional to said range scale.
 7. The invention of claim 6 whereinsaid chirp bandwidth is given by: S=c/(2·R _(MAX) ·q)  [2]where ‘c’ isthe speed of light and ‘q’ is the fractional range resolution.
 8. Adata-processing system adapted for use with a system for scanning a beamincluding a plurality of pulses of electromagnetic energy, receivingreflections of the pulses in the beam as return signals, processing thereturn signals to extract range and range rate measurements, processingthe range and range rate measurements to form a plurality of range bins,and selecting and weighting return pulses in the range bins, thedata-processing system comprising: first means for performing a FastFourier Transform for a predetermined number of pulses in at least oneof the range bins at at least one frequency; second means for performinga Fast Fourier Transform for pixels of azimuth data across the rangebins; and third means for applying ambiguity nulling weights to eachpixel of data in each range bin.
 9. The invention of claim 8 wherein thethird means includes means for applying nulling weights to each pixel ofdata in each range bin based on beam scan geometry.
 10. The invention ofclaim 8 wherein the third means includes means for applying nullingweights to each pixel of data in each range bin based on scan history.11. The invention of claim 8 wherein the third means includes means forapplying nulling weights to each pixel of data in each range bin basedon range.
 12. The invention of claim 8 wherein the third means includesmeans for applying nulling weights to each pixel of data in each rangebin based on range ambiguity.
 13. The invention of claim 8 wherein thethird means includes means for applying nulling weights to each pixel ofdata in each range bin based on Doppler ambiguity.
 14. The invention ofclaim 13 wherein the third means includes means for applying nullingweights to each pixel of data in each range bin based on pulserepetition frequency.
 15. The invention of claim 8 wherein the thirdmeans includes means for applying nulling weights to each pixel of datain each range bin at each of a plurality of predetermined frequencies.16. The invention of claim 15 further including fourth means forperforming pulse detection integration across each of the frequencies ofthe beam.
 17. A system comprising: first means for scanning a beamincluding a plurality of pulses of electromagnetic energy; second meansfor receiving reflections of the pulses in the beam as return signals;third means for processing the return signals to extract range and rangerate measurements therefrom; fourth means for processing the range andrange rate measurements to form a plurality of range bins; fifth meansfor selecting and weighting return pulses in the range bins; sixth meansfor performing a Fast Fourier Transform for a predetermined number ofpulses in at least one of the range bins at at least one frequency;seventh means for performing a Fast Fourier Transform for pixels ofazimuth data across the range bins; and eighth means for applyingnulling weights to each pixel of data in each range bin.
 18. Theinvention of claim 17 wherein the beam is a radar beam.
 19. Theinvention of claim 17 wherein said first means includes means foroutputting a beam excited by a waveform having a burst width and pulsewidth scaled proportionately with a selected range scale.
 20. Theinvention of claim 17 wherein the fifth means for selecting andweighting return pulses in the range bins includes means for selectingand weighting the return pulses to reduce sidelobes resulting from theFast Fourier transform of the predetermined number of pulses.
 21. Theinvention of claim 17 wherein the fifth means includes means forselecting the pulses based on antenna scan weight.
 22. The invention ofclaim 17 wherein the fifth means includes means for selecting the pulsesbased on a pulse repetition frequency of the pulses.
 23. The inventionof claim 17 wherein the seventh means includes means for selecting FastFourier Transform weighting windows.
 24. The invention of claim 17wherein the seventh means further includes means for performing a fastFourier transform for pixels of azimuth data across the range bins basedon scan geometry.
 25. The invention of claim 24 wherein the seventhmeans further includes means for performing a fast Fourier transform forpixels of azimuth data across the range bins based on a scan history ofthe beam.
 26. The invention of claim 17 wherein the eighth meansincludes means for applying nulling weights to each pixel of data ineach range bin based on beam scan geometry.
 27. The invention of claim17 wherein the eighth means includes means for applying nulling weightsto each pixel of data in each range bin based on scan history.
 28. Theinvention of claim 17 wherein the eighth means includes means forapplying nulling weights to each pixel of data in each range bin basedon range.
 29. The invention of claim 17 wherein the eighth meansincludes means for applying nulling weights to each pixel of data ineach range bin based on range ambiguity.
 30. The invention of claim 17wherein the eighth means includes means for applying nulling weights toeach pixel of data in each range bin based on Doppler ambiguity.
 31. Theinvention of claim 30 wherein the eighth means includes means forapplying nulling weights to each pixel of data in each range bin basedon pulse repetition frequency.
 32. The invention of claim 17 wherein theeighth means includes means for applying nulling weights to each pixelof data in each range bin at each of a plurality of predeterminedfrequencies.
 33. The invention of claim 32 further including ninth meansfor performing pulse detection integration across each of thefrequencies of the beam.
 34. A method including the steps of: scanning abeam including a plurality of pulses of electromagnetic energy;receiving reflections of the pulses in the beam as return signals;processing the return signals to extract range and range ratemeasurements therefrom; processing the range and range rate measurementsto form a plurality of range bins; selecting and weighting return pulsesin the range bins; performing a Fast Fourier Transform for apredetermined number of pulses in at least one of the range bins at atleast one frequency; performing a Fast Fourier Transform for pixels ofazimuth data across the range bins; and applying nulling weights to eachpixel of data in each range bin.
 35. The invention of claim 34 whereinthe step of scanning the beam includes the step of outputting a beamexcited by a waveform having a burst width and pulse width scaledproportionately with a selected range scale.
 36. The invention of claim34 wherein the step of selecting and weighting return pulses in therange bins includes the step of selecting and weighting the returnpulses to reduce sidelobes resulting from the Fast Fourier transform ofthe predetermined number of pulses.
 37. The invention of claim 34wherein the step of selecting and weighting return pulses in the rangebins includes the step of selecting the pulses based on antenna scanweight.
 38. The invention of claim 34 wherein the step of selecting andweighting return pulses in the range bins includes the step of selectingthe pulses based on a pulse repetition frequency of the pulses.
 39. Theinvention of claim 34 wherein the step of performing a Fast FourierTransform for pixels of azimuth data across the range bins includes thestep of selecting Fast Fourier Transform weighting windows.
 40. Theinvention of claim 34 wherein the step of performing a Fast FourierTransform for pixels of azimuth data across the range bins furtherincludes the step of performing a fast Fourier transform for pixels ofazimuth data across the range bins based on scan geometry.
 41. Theinvention of claim 40 wherein the step of performing a Fast FourierTransform for pixels of azimuth data across the range bins furtherincludes the step of performing a fast Fourier transform for pixels ofazimuth data across the range bins based on a scan history of the beam.42. The invention of claim 34 wherein the step of applying nullingweights to each pixel of data in each range bin includes the step ofapplying nulling weights to each pixel of data in each range bin basedon beam scan geometry.
 43. The invention of claim 34 wherein the step ofapplying nulling weights to each pixel of data in each range binincludes the step of applying nulling weights to each pixel of data ineach range bin based on scan history.
 44. The invention of claim 34wherein the step of applying nulling weights to each pixel of data ineach range bin includes the step of applying nulling weights to eachpixel of data in each range bin based on range.
 45. The invention ofclaim 34 wherein the step of applying nulling weights to each pixel ofdata in each range bin includes the step of applying nulling weights toeach pixel of data in each range bin based on range ambiguity.
 46. Theinvention of claim 34 wherein the step of applying nulling weights toeach pixel of data in each range bin includes the step of applyingnulling weights to each pixel of data in each range bin based on Dopplerambiguity.
 47. The invention of claim 46 wherein the step of applyingnulling weights to each pixel of data in each range bin includes thestep of applying nulling weights to each pixel of data in each range binbased on pulse repetition frequency.
 48. The invention of claim 34wherein the step of applying nulling weights to each pixel of data ineach range bin includes the step of applying nulling weights to eachpixel of data in each range bin at each of a plurality of predeterminedfrequencies.
 49. The invention of claim 48 further including the step ofperforming pulse detection integration across each of the frequencies ofthe beam.